A process and an apparatus for removing nitrogen oxides from gases are known from DE-OS 3,642,018. As specified in detail there, the zeolite catalyst, which can be formed as a monolithic catalyst or as a bulk material catalyst, can preferably contain zeolites of types X, Y or mordenite. The mode of operation of this known process is based on the fact that the nitrogen oxides contained in the exhaust gases, for example of a motor vehicle internal combustion engine, can be reacted with the hydrocarbons also normally present in the exhaust gases, as reducing agents, and the oxygen present during the combustion of a lean fuel-air mixture. With that the known addition of ammonia as reducing agent, which is problematic for several reasons, especially in motor vehicles, can be dispensed with.
A precondition for the described operation of the known process is consequently a definite minimum value of the hydrocarbon concentration in the exhaust gas, since these, as described, serve as reducing agents. Accordingly the known process cannot be used, at least optimally, in the cases in which, as with diesel engines or Otto engines with direct gasoline injection, the hydrocarbon concentration in the exhaust gas is definitely less than the nitrogen oxide concentration.
In German application P 3,830,045.1 this problem is solved by adding urea to the exhaust gas stream containing oxygen in excess, whereby the addition and storage of problematic reducing agents such as ammonia is avoided.
In the course of this, urea reduces the nitrogen oxides in an exhaust gas stream containing oxygen in excess on a catalyst. This is no doubt based on the fact that urea (H.sub.2 N--CO--NH.sub.2) hydrolyzes easily to CO.sub.2 and 2 NH.sub.3 or decomposes with formation of NH.sub.2 radicals.
In the final analysis, urea is thus a reservoir for a reducing agent. It is true that for treating the exhaust gases emitted by a motor vehicle internal combustion engine, for example, the reducing agent must be brought along too in a tank fastened to the vehicle, but it exists in unproblematic form in the shape of aqueous urea solution, and the reducing agent is only formed later.
The metering of the reducing agent can be carried out by dosing 50% to 200% of the theoretically required amount, preferably 80% to 100% of the required amount.
On a Cu-exchanged zeolite of the ZSM type (ZSM-5) the following NO.sub.x reduction, for example, was achieved with urea:
TABLE 1 ______________________________________ NO.sub.x : urea 1:0.25 1:0.41 1:0.52 1:0.83 NO.sub.x : conversion % 50% 74% 83% 99% ______________________________________
The addition of reducing agent will accordingly be controlled or adjusted according to the mode of operation or operating quantities of the apparatus producing the exhaust gas, thus for example a vehicle internal combustion engine. For example with a vehicle internal combustion engine there is when driving a definite nitrogen oxide concentration in the exhaust gas as a function of the operating point of the machine at the moment, which is defined for example by engine speed and load, and the control or adjustment of the addition of reducing agent is carried out so that the exhaust gas after the zeolitic catalyst in the direction of flow contains the lowest possible proportions of reducing agent and nitrogen oxides. In the exemplary test it is assumed that one mole urea forms two moles of a reducing agent. The dosage of urea was so chosen that about 90% of the theoretically required amount of reducing agent was available. The space velocity was so chosen that 12,900 h.sup.-1 was reached. However, at low exhaust gas temperatures, such as are common in the start-up phase with diesel engines, the Cu-containing zeolitic catalysts claimed in the prior application reach only small conversion rates. But it is desirable to achieve as high as possible a conversion immediately in the start-up phase.